The present invention relates to a multi-piece golf ball which is composed of a core and outer layers contiguous to each other including at least one layer of non-ionomer thermoplastic elastomer and at least one layer of ionomer. The golf ball is characterized by good adhesion between the outer layer of non-ionomer thermoplastic elastomer and the outer layer of ionomer. It is also characterized by outstanding durability, rebound resilience, and productivity.
There has recently been proposed a new golf ball with an outer layer made of a soft non-ionomer thermoplastic elastomer typified by thermoplastic polyurethane elastomer. The golf ball with an outer layer of non-ionomer thermoplastic elastomer is superior in spin performance and shot feeling to conventional golf balls with an outer layer of ionomer. Thus the outer layer of non-ionomer thermoplastic elastomer is becoming predominant.
On the other hand, improvements are being made on the outer layer of ionomer for better rebound resilience, by replacing the conventional ionomer with a highly neutralized ionomer (which is obtained by incorporating an ionomer resin with a large amount of fatty acid and neutralizing ion source). The highly neutralized ionomer has a much better rebound resilience than the conventional ionomer. Thus the combination of an outer layer of polyurethane and an outer layer of highly neutralized ionomer has helped create a practical multi-piece golf ball having both ideal flying performance and spinning performance. These golf balls are disclosed in specification of U.S. Pat. Nos. 3,395,109, 4,248,432, 4,442,282, Japanese publication of (unexamined) patent application No. 9-271538 and No. 11-178949.
Unfortunately, an ionomer resin has a higher polarity than an ordinary non-ionomer thermoplastic elastomer and hence it is poor in adhesion to other resins than ionomer. Because of this poor adhesion, a multi-piece golf ball which has in combination an outer layer of ionomer and an outer layer of non-ionomer thermoplastic elastomer is liable to decrease in durability and rebound resilience.
One countermeasure against this trouble is to form a primer layer between the outer layer of ionomer and the outer layer of non-ionomer thermoplastic elastomer. However, it needs an additional step for primer coating, which leads to an increase in production cost.